This invention relates to bulk ionically conductive polymer gels and their preparation, and to galvanic cells containing them.
The most commonly used electrolytes are fluid liquids which comprise solutions in a liquid solvent of solute ionic species. Such fluid liquid electrolytes, on incorporation into a galvanic cell, permit migration of ions between the electrodes of the cell and, as a consequence, the provision of electric free energy to a closed external circuit. Despite their widespread use, such electrolytes nonetheless suffer from several disadvantages. Thus, they are often corrosive, leading to leakage from cells and they do not provide a firm barrier between the electrodes when required to assist in stabilizing the inter-electrode distance and in preventing physical loss of electrode material from the electrode surface.
In order, in part, to overcome the disadvantages inherent in fluid liquid electrolytes, particularly in relation to galvanic cells, considerable effort has been expended in attempts to provide solid or highly viscous polymeric electrolytes which contain salts which display mobility, under appropriate conditions, of at least some of the ionic species present. The solid polymeric electrolytes are capable of acting in thin film form as electrode separators and in solid-state cells can deform while maintaining good contact with the electrodes, thus minimizing problems arising from mechanical strain arising either from mechanical stresses during use or volume changes during the charge/discharge cycle. A particular area of importance is in cells that do not depend upon water as a component of the electrolyte, such as lithium cells where water and other materials capable of reacting with lithium are undesirable. The potential uses for such materials are not limited to batteries but include, inter alia, sensor devices and thermoelectric energy convectors.
A prominent polymeric material for this purpose has been poly(ethylene oxide) (PEO), in which certain salts are soluble and can form complexes. The electrical and mechanical properties of such polymer electrolyte materials, although encouraging, require further enhancement before commercialisation can be envisaged. Improvements in the properties have been obtained using graft copolymers in which short poly(ethylene oxide) chains are present as pendant units attached to a long main chain. Such materials have been described in GB-A-2161488. Another means of improving the mechanical properties is to use block copolymers in which short poly(ethylene oxide) chains alternate with other units such as polysiloxane. Yet another means is to cross-link a poly(ethylene oxide) with an epoxy compound. In each case the polymer electrolyte contains a suitable salt complexed with the polymer to provide the ionic species required for conductivity. In all these cases the conductivities reported at 25xc2x0 C. or at room temperature are at best about 10xe2x88x924 Siemens per cm. These values are an order of magnitude less than a commonly cited target for commercial realization of 10xe2x88x923 Siemens per cm.
It is also possible to provide polymer electrolytes which consist of a mixture of a polymer, preferably of high molecular weight, with a compound of low molecular weight that is a solvent for the polymer in the range of temperatures in which the electrolyte is to be sued, together with an appropriate salt that is soluble in the polymer and in the compound of low molecular weight. For example, as disclosed in GB-A-2212504 and 2216132, polymer electrolytes consisting of poly-N,N-dimethylacrylamide or closely related poly-N-substituted acrylamide of high molecular weight plasticized with dimethylacetamide together with lithium trifluoromethane sulphonate (lithium triflate) as the salt component have been evaluated and found to exhibit good conductivities together with good mechanical properties. These polymer electrolytes are gel-like in character, but the compound of low molecular weight must not exceed a certain limiting concentration above which the system loses its gel-like character and begins to flow. The ionic conductivity is higher at the higher concentrations of the compound of low molecular weight, but the material becomes increasingly more flexible. Conductivities of 7xc3x9710xe2x88x923Scmxe2x88x921 at 20xc2x0 C. are obtainable but this requires at least 60% or more of the low molecular weight compound and at this level the mechanical properties are poor. It has proved possible by cross-linking the polymer to improve the mechanical properties to a useful level with as much as 80% of the low molecular weight compound present, and thus to obtain conductivities at 20xc2x0 C. exceeding 10xe2x88x923Scmxe2x88x921. These products may prove of commercial interest, but the process for making the cross-linked polymer electrolyte film is somewhat complex for convenient incorporation into a process for cell manufacture.
This invention seeks to provide ionically conductive materials that provide high bulk tonic conductivities at ambient temperature together with good mechanical properties.
According to one aspect of the invention there is provided an ionically conductive, ion-containing gel having a bulk ionic conductivity at 20xc2x0 and 10 kHz greater than 10xe2x88x924 Siemens per centimetre and a dynamic modulus at 10 Hz greater than 103 Pa. preferably greater than 104 Pa, e.g.  greater than 105 Pa, wherein the gel consists of a minor amount of a crystallizable polymer such as a polyester, a major amount of an organic compound that is a solvent for a salt at 20xc2x0 C. but is not a solvent for the crystallizable polymer at 20xc2x0 C., and a salt dissolved in the organic compound at a concentration greater than 4% by mass based on the organic compound. The said minor amount is up to 50% by mass, preferably up to 40%, e.g. at least 5% such as at least 10%, typically 20-30%.
The ion-containing gels of this invention can provide better ionic conductivities both an ambient and elevated temperatures than polymer electrolytes based on polymer-salt complexes previously described and better mechanical properties than polymer electrolytes of good ionic conductivity based on polymer-salt-plasticizing solvent complexes previously described.
The ion-containing gels of this invention can normally be regarded as thermoreversible gels in which the junctions are physical associations, possibly corresponding with crystal structures comprising only a small portion of the polymer chains.
The crystallizable polymer may itself be capable of complexing with the salt through containing, for example, ether or amide groups, but it is not essential that the crystallizable polymer should dissolve or complex with the salt. This contrasts with previously described ion-conducting electrolyte systems based upon polymers where it has been essential that the polymer should dissolve or complex the salt and desirable that the polymer should be non-crystallizable.
Suitable crystallizable polymers for use in this invention include crystallizable polyesters such as poly(ethylene terephthalate), poly(1,4-butylene terephthalate) and poly(3-oxybutanoate), crystallizable polyamides such as poly (hexamethylene adipamide) and poly(m-phenylene isophthalamide), crystallizable polyethers and crystallizable substituted (e.g. halo) polyolefins such as substituted polyvinylidenes. Further examples include polyhydroxybutyric acid, poly(metaxylylene adipamide), poly(vinylidene fluoride), polyoxymethylene and polyoxyethylene. The crystallizable polymer is normally dissolved at a high temperature in the other components and can provide the required mechanical rigidity for the product at lower temperatures. If inadequate crystallizable polymer is present, the mechanical properties and dimensional stability will suffer. The crystallizable polymer is preferably of a sufficiently high molecular weight to form coherent films and fibres. In general, the higher the molecular weight of the polymer, the better the mechanical properties of the gel structure formed and the lower the concentrate of the polymer required to maintain a gel structure, and the lower the concentration of polymer, the higher the conductivity.
Suitable organic compounds that are solvents for a salt at 20xc2x0 C. but are not solvents for the crystallizable polymer at 20xc2x0 C. include amides (preferably tertiary amides) which may be cyclic such as dimethyl formamide, dimethyl acetamide, N-methyl-2-pyrrolidinone and N-formyl piperidine, sulphoxides and ethers (preferably polyfunctional) such as the dimethyl ethers of diethylene glycol, triethylene glycol and tetraethylene glycol. Mixtures of such compounds may also be used. Where its more modest oxidation-reduction stability is adequate, the solvent organic compound may be dimethyl sulphoxide. It will be understood that these compounds do become solvents for the crystallisable polymer at some temperature above 20xc2x0 C., e.g. above 100xc2x0 C. or above 150xc2x0 C. For use in batteries it is preferable that the organic compounds should be free from chemical groups that can react with electrode components. Thus for lithium batteries the organic compounds should not contain hydroxyl groups and should be as free of water as possible.
Suitable salts include alkali metal salts such as salts of lithium, sodium or potassium and substituted or unsubstituted ammonium. Lithium is particularly preferred because of the high solubility of many lithium salts in suitable organic compounds and the importance of lithium as an electrode material. The counterbalancing anion is preferably large and preferably a weak conjugate base. Examples include the monovalent anions derived from higher halogens and pseudohalogens, for example Brxe2x88x92, Ixe2x88x92 and SCNxe2x88x92 and complex inorganic, carboxylic and sulphonic, preferably perfluorinated alkyl carboxylic and sulphonic, monovalent anions, for example ClO4xe2x88x92, HgI3xe2x88x92, BF4xe2x88x92, CF3COOxe2x88x92, and CF3SO3xe2x88x92. The concentration of salt based in the organic compound should be greater than 4% by weight and is limited at the upper end of the range by a saturation solubility of the salt in the organic compound in the presence of the polymer. The salt Is preferably present in the gel structure at a concentration such that it does not exceed its saturation solubility throughout the proposed temperature range of use. Hence, for each combination of organic compound and salt and intended temperature there is an optimum concentration of salt for the highest conductivities to be obtained.
Gels according to this invention may be prepared by forming a solution of the polymer in the organic compound at above 20xc2x0 C. (preferably above 100xc2x0 C. such as above 150xc2x0 C.), incorporating the salt into the solution either by addition after it has been formed or simultaneously or preferably by solution in the organic compound before the addition of the polymer, then cooling the solution. Such cooling will be understood to be to a temperature below the critical solution temperature of the polymer in the mixture of the organic compound and the salt.
The present invention also provides a galvanic cell wherein the electrolyte comprises an ionically conductive gel as herein defined; and a battery of such cells.